F-theta objective

ABSTRACT

An F-theta objective is provided that has five individual lenses with a first individual lens as biconcave lens with a focal length, a second individual lens as meniscus with a focal length, a third individual lens as meniscus with a focal length, a fourth individual lens as a biconvex lens with a focal length and a fifth individual lens as a plano-convex lens with a focal length and a total focal length f of the F-theta objective. A ratio between the focal lengths to the total focal length satisfies predetermined conditions.

This nonprovisional application claims priority under 35 U.S.C. §119(a)to German Patent Application Nos. DE 20 2013 006 369.6, which was filedin Germany on Jul. 16, 2013; DE 20 2013 009 184.3, which was filed inGermany on Oct. 17, 2013, and to DE 20 2014 002 322.0, which was filedin Germany on Mar. 14, 2014, all of which are herein incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an F-theta objective as can be used in a scanapparatus for laser material processing.

2. Description of the Background Art

An F-theta objective focuses a laser beam, which is incident in a mannersuch that it scans over a scan-angle region +/−θ with respect to theoptical axis of the F-theta objective, into a flat image field, wherein,within this scan-angle region, the ratio of scan angle and distance ofthe point of incidence of the laser beam from the optical axis in theimage field follows a linear function. That is to say that a laser beamscanning at a constant angular velocity generates a focal point in theimage field, which focal point moves at a constant speed. Here, the sizeof the focal point should be constant at each location in the imagefield.

The size of the focal point is determined in dependence on the purposeof the laser material processing, e.g. writing, coating removal orcutting.

Owing to the wavelength-dependent refraction of the laser beam as itpasses through the F-theta objective, F-theta objectives are corrected,in order to achieve a high focal point quality, to the wavelength of theprocessing laser beam used, that is to say the objective is calculatedsuch that it has, for an image field of prespecified size within apermissible temperature tolerance for a prespecified wavelength and aprespecified laser beam diameter, no or only very slight opticalaberrations which result in a distinct change in the size of the focalpoint. In particular for use in laser material processing, F-thetaobjectives have a large image field and a large total focal length.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an F-thetaobjective for high-power lasers with a wavelength of 355 nm and a totalfocal length of between 90 mm and 110 mm, in particular 100 mm-105 mm,which enables imaging of the laser radiation without retroreflection andshould be of a telecentric configuration. It is thus possible, forexample, for holes to be inserted perpendicularly into a workpieceand/or to achieve identical diameters in the X- and Y-scanningdirections [otherwise ellipse in X or Y]. At the same time, a uniformburr formation is intended to be achieved thereby during materialprocessing. The aim is therefore to obtain a higher energy density witha circular focus.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingwhich is given by way of illustration only, and thus, is not limitive ofthe present invention, wherein the sole FIGURE illustrates a geometricoptics diagram of an objective according to an embodiment of theinvention.

DETAILED DESCRIPTION

An F-theta objective according to an embodiment of the invention has,arranged at a distance upstream of the F-theta objective, an entrancepupil EP which can be arranged in a plane in which a scanner mirror, orif two scanner mirrors are used a substitute plane that is calculatedfor this, is located, and comprises five individual lenses L₁-L₅ whichare arranged on a common optical axis A. The five individual lensesL₁-L₅ are configured such that they form a“negative-positive-positive-positive-positive” lens sequence. That is tosay that the first individual lens L₁ has a negative focal length,whereas the second to fifth lenses L₂-L₅ have a positive focal length.

The first lens L₁ here is a biconcave lens, the second lens L₂ is ameniscus, the third lens L₃ is also a meniscus, the fourth lens L₄ is abiconvex lens, and the fifth lens L₅ is a plano-convex lens.

The focal lengths of the five individual lenses should satisfy thefollowing requirement:

The focal length ratio of the focal length f₁ of the first lens L₁ tothe total focal length is f: −1.0<f₁/f<−0.4.

The focal length ratio of the focal length f₂ of the second lens L₂ tothe total focal length is f: +3.6<f₂/f<+4.2.

The focal length ratio of the focal length f₃ of the third lens L₃ tothe total focal length is f: +2.0<f₃/f<+2.8.

The focal length ratio of the focal length f₄ of the fourth lens L₄ tothe total focal length is f: +1.4<f₄/f<+1.8.

The focal length ratios of the focal length f₅ of the fifth lens L₅ tothe total focal length is f: +2.7<f₅/f<+3.1.

A protective glass SG can be arranged downstream of the five individuallenses L₁-L₅.

The useful effect achieved by an F-theta objective according to theinvention is that with five individual lenses L₁-L₅ of quartz glass, anF-theta objective with a total focal length of between 90 mm and 110 mm,which is suitable for a high-power laser and is corrected for awavelength of 355 nm, results. The F-theta objective is configured withits parameters such that retroreflections, which are produced on theoptically active faces of the lenses L₁-L₅ and, if appropriate, theprotective glass SG, are not focused onto the optical surfaces of thelenses L₁-L₅ or onto the mirrors arranged around the entrance pupil EP.The configuration is adapted such that no retroreflections occur in thematerial of the lenses and no retroreflections occur at the sites of thescanner positions.

The specific structure and the parameters of an exemplary embodiment forsuch an F-theta objective are described below.

The entrance pupil EP of the F-theta objective is located at a distanced₁ in front of the front vertex point of the first lens L₁, a biconcavelens with a thickness d₂, the front surface of which has a radius r₁ andthe back surface of which has a radius r₂. This first lens L₁ isfollowed, with an air gap d₃, by the second lens L₂, a positive meniscushaving a thickness d₄, the front surface of which has a radius r₃ andthe back surface of which has a radius r₄.

This is followed, with an air gap d₅, as third lens L₃, likewise by apositive meniscus having a thickness d₆, the front surface of which hasa radius r₅ and the back surface of which has a radius r₆. This isfollowed, with an air gap d₇, by the fourth lens L₄, a biconvex lenshaving a thickness d₈, the front surface of which has a radius r₇ andthe back surface of which has a radius r₈. This is followed, with an airgap d₉, by a plano-convex lens having a thickness of d₁₀, the frontsurface of which has a radius r₉ and the back surface of which has aradius r₁₀. In the exemplary embodiment, this is followed, with an airgap d₁₁, by a plane-parallel protective glass SG having a thickness ofd₁₂. The image field BE is produced at a distance of d₁₃ from theprotective glass SG. Quartz glass with a refractive index n_(e) wasselected as the material for all of the individual lenses L₁-L₅ and theprotective glass SG.

The radii of the individual lenses L₁-L₅, and the thicknesses anddistances d thereof are given in the following table 1.

Medium Radius (mm) Thickness (mm) n_(e) air EP d₁ 35.0 L₁ r₁ 38.8 d₂ 3.01.46 L₁ r₂ 243.6 air d₃ 5.5 1 L₂ r₃ 118.8 d₄ 15.0 1.46 L₂ r₄ 76.5 air d₅2.2 1 L₃ r₅ 285.8 d₆ 20 1.46 L₃ r₆ 86.0 air d₇ 0.2 L₄ r₇ 1695.9 d₈ 251.46 L₄ r₈ 90.2 air d₉ 0.2 L₅ r₉ 138.0 d₁₀ 15 1.46 L₅ r₁₀ □ air d₁₁ 5 SGr₁₁ □ SG r₁₂ □

The distances and thicknesses are all designated d and are numberedaccording to their sequence along the optical axis A of the F-thetaobjective in the beam passage direction and indicated as d₁-d₁₃ in FIG.1.

The terms “front” and “back” surface can refer to the beam passagedirection. The radii r₁-r₁₂ can be associated clearly with reference tothe relevant lenses L₁-L₅ and the protective glass SG and are thereforenot indicated in FIG. 1 for the sake of clarity.

In dependence on the material-dependent refractive indices n_(e) of theindividual lenses L₁-L₅, the thicknesses d₂, d₄, d₆, d₈, d₁₀ of theindividual lenses L₁-L₅ determine, in connection with the radii ofcurvature r₁-r₁₀ of the individual lenses, in each case the focallengths f₁-f₅ of the individual lenses L₁-L₅. The focal lengths f₁-f₅,which in each case describe the distance of a focal point from a mainplane of an individual lens L₁-L₅, are not shown in FIG. 1, because themain planes of the individual lenses L₁-L₅ are not indicated for thesake of clarity. The total focal length f, which describes the distanceof the image field BE from a substitute main plane for the F-thetaobjective, is likewise not indicated.

For an F-theta objective with the parameters indicated here, the resultis a focal length f₁ with respect to the total focal length f of −0.7for the first lens L₁, a focal length f₂ with respect to the total focallength f of +3.9 for the second lens L₂, a focal length f₃ with respectto the total focal length f of +2.4 for the third lens L₃, a focallength f₄ with respect to the total focal length f of +1.6 for thefourth lens L₄ and a focal length f₅ with respect to the total focallength f of +2.9 for the fifth lens L₅.

The result of the arrangement of the lenses L₁-L₅ successively inconjunction with their air gaps d₃, d₅, d₇, d₉ is the total focal lengthf of 103 mm. The F-theta objective is corrected for a wavelength of 355nm.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are to beincluded within the scope of the following claims.

What is claimed is:
 1. An F-theta objective, comprising five individuallenses sequentially arranged, comprising: a first individual lens L₁ asa biconcave lens with a focal length f₁, a second individual lens L₂ asa meniscus with a focal length f₂; a third individual lens L₃ as ameniscus L₃ with a focal length f₃; a fourth individual lens L₄ as abiconvex lens with a focal length f₄; and a fifth individual lens L₅ asa plano-convex lens with a focal length f₅, wherein the F-thetaobjective has a total focal length f, and wherein the ratio between thefocal lengths f₁-f₅ to the total focal length f satisfies the followingconditions:−1.0<f ₁ /f<−0.4+3.6<f ₂ /f<+4.2+2.0<f ₃ /f<+2.8+1.4<f ₄ /f<+1.8+2.7<f ₅ /f<+3.1.
 2. The F-theta objective as claimed in claim 1,wherein f₁/f=−0.7, f₂/f=+3.9, f₃/f=+2.4, f₄/f=+1.6, f₅/f=+2.9.
 3. TheF-theta objective as claimed in claim 2, wherein an entrance pupil EP ofthe F-theta objective is located in front of the first individual lensL₁ at a distance d₁ of 35.0 mm.
 4. The F-theta objective as claimed inclaim 1, wherein a scanned beam is aligned telecentrically with respectto an optical axis.